Large bowel diarrhea in cats results from a variety of infectious and noninfectious causes. The association between feline large bowel diarrhea and trichomonad infection has only recently been recognized. Historically, trichomonads were believed to be opportunistic parasites whose numbers increased dramatically in the presence of altered large bowel physiology.

A trichomonad suspected of being Pentatrichomonas hominis was isolated from the stools of cats with large bowel diarrhea. Koch's postulates were fulfilled when trichomonads from the stools of a naturally infected cat reproduced the disease in specific pathogen-free cats. However, confusion about the identity of the pathogen remained. The study reported here used rRNA gene sequence analysis; restriction enzyme digest mapping; and light, transmission, and scanning electron microscopy to determine the identity of the organism. Trichomonads isolated from three domestic cats with diarrhea were used in the studies.

The morphologic and genetic analysis of the feline trichomonads in this study showed that they were indistinguishable from Tritrichomonas foetus. This organism is a known venereal pathogen of the reproductive tract in cows and has been reported in a case of fatal meningitis in an immunosuppressed person. Cats with trichomonad diarrhea often have cow pie stools and fecal incontinence resulting in contamination of the hair coat and environment. The zoonotic potential is unknown but should be investigated. The role of cattle in the ecology and epidemiology of feline T. foetus infection is unknown and should also be studied. The trichomonads in human or feline stools may be mistaken for Giardia trophozoites, delaying diagnosis. Currently, chemotherapeutic agents have not been reported to be effective against either feline or bovine isolates.

COMMENTARY: Because of the zoonotic potential of these organisms, proper diagnosis is important. The history often includes waxing and waning diarrhea. The diarrhea may respond to antimicrobial therapy with a return of clinical signs shortly after antimicrobials are discontinued. The organism may be seen on a direct saline smear, and the feces must be fresh. T. foetus is spindle- or pear-shaped. The undulating membrane may be visible. The organisms must be differentiated from the binucleated Giardia trophozoite, which is more sluggish.

Tritrichomonas foetus and not Pentatrichomonas hominis is the etiologic agent of feline trichomonal diarrhea. Levy MG, Gookin JL, Poore M, et al. J PARASITOL 89:99-104, 2003.